1. Field of the Invention
The invention relates to an exposure system for use in forming inorganic luminescent material (phosphors) layers in a plasma display panel (referred to hereinafter as PDP) in color, which is an emissive type flat-panel display utilizing electrical gas discharges, and a method of forming the phosphor layers using the exposure system.
2. Description of the Related Art
A PDP generally has a construction wherein two glass sheet substrates, each provided with a set of electrodes regularly arranged thereon, are disposed facing each other, and gases comprising mainly Ne, Xe, and the like are enclosed therebetween. Electrical discharges are caused to occur in minuscule cells disposed in close proximity of the electrodes when a voltage is applied between the sets of the electrodes, each cell emitting light for display. For display of information, the electrical discharges are caused to occur selectively at the respective cells arranged in a regular fashion so that light is emitted accordingly. There are two types of PDPs, one being a direct current (DC) type with the electrodes exposed to a discharge space, and the other an alternating current (AC) type with the electrodes covered by insulation layers. Both types are further classified into a refresh driving type and a memory driving type.
FIG. 1 is a view illustrating the construction of the AC type PDP by way of example, showing a perspective view thereof in a condition wherein a front sheet is separated from a rear sheet for convenience. As shown in the figure, two glass substrates 1 and 2, disposed in parallel with, and opposite to each other, are held at a predetermined interval by barrier ribs 3 arranged in parallel with each other on the glass substrate 2 serving as the rear sheet. Composite electrodes composed of a transparent electrode 4 for holding up electrical discharges and a metallic bus electrodes 5 are arranged in parallel with each other on the back surface of the glass substrate 1 serving as the front sheet, and a dielectric layer 6 is formed so as to cover the composite electrodes. Further, a protective layer 7 (MgO layer) is formed on top of the dielectric layer 6. On the front surface of the glass substrate 2 as the rear sheet, address electrodes 8 are formed in parallel with each other, and disposed between the barrier ribs 3 so as to cross the composite electrodes 5 at right angles. A phosphor substance layer 9 is provided covering sidewall surfaces of the barrier ribs 3 and the bottom surfaces of the cells. The AC type PDP is of a surface discharge type, and constructed such that electrical discharges are caused to occur in an electrical field set up in space when an a-c voltage is applied between the composite electrodes provided on the front sheet. In this case, the direction of the electric field to which the a-c voltage is applied changes according to frequency of the a-c. Ultraviolet radiation resulting from the electrical discharges causes the phosphor substance layer 9 to emit light so that light transmitting through the front sheet can be visually recognized by viewers.
In the PDP described above, the rear sheet is fabricated by forming the address electrodes 8 on the glass substrate 2 first, forming the dielectric layer so as to cover same if necessary, forming the barrier ribs 3, and then providing phosphor layers, composed of the phosphor substance layer 9, between the barrier ribs facing each other. It is well known that the electrodes 8 are formed by patterning using the photolithographic techniques on an electrode material film formed on the glass substrate 2 by use of the vacuum deposition method, sputtering method, plating method, thick film techniques, and the like, or by patterning on a thick film paste using the screen printing method. Further, the dielectric layer is formed by the screen printing method, or the like, and the barrier ribs 3 are formed by overlap printing using the screen printing method, or by the sandblasting method, or the like. The phosphor layers are formed by a method of selectively filling up between the barrier ribs 3 with phosphor paste in three colors, red (R), green (G), and blue (B), by use of the screen printing method.
As described in the foregoing, the method of filling up directly between the barrier ribs facing each other with the phosphor paste in three colors by use of the screen printing, and thereafter, heat treating same is adopted for formation of the phosphor layers between the barrier ribs. However, there have been problems that it is difficult to manufacture screen frames for substrates in large sizes while deviation in size occurs, and it is also difficult to ensure accuracy in the case of high resolution types. Accordingly, a method of forming the phosphor layers by applying the photolithographic techniques to a photosensitive phosphor paste or a photosensitive phosphor film has been contemplated. Use of an exposure method utilizing collimated rays of light in such a case has been under study.
It is to be pointed out, however, that the exposure system utilizing collimated rays of light requires an optical system as shown in FIG. 2 to produce collimated rays of light accurate enough for forming a pattern consisting of lines and spaces on the order of several to several tens .mu.m, leading to a higher cost of the exposure system itself. That is, in the optical system, rays of light emitted from a light source 10 are not directly radiated towards a work substrate 11, but adjusted by use of a reflective lens 12 and a collimating lens 13 such that intensity of radiation becomes uniform within the surface of the work substrate 11.
There is also a problem that it is difficult to form the phosphor layers in a desirable shape with the collimated rays of light. More specifically, in the photolithographic techniques, first phosphor layer forming layers are formed by coating throughout the work substrate with the barrier ribs formed thereon with a photosensitive phosphor paste and subsequently, drying same, or by heating and fitting by pressure a photosensitive phosphor film onto the work substrate with the barrier ribs formed thereon. Thereafter, the phosphor layers are formed by exposing via a photomask and developing the phosphor layer forming layer. This process is applied in a similar manner to the phosphor layer forming layers in different colors, forming the phosphor layers in three colors and heat treating same in the last step of the process. It is normally a desirable practice in the process to design a photomask 23 having openings in width matching an interval between barrier ribs 21 as shown in FIG. 3 for exposing a phosphor layer forming layer 22 between the barrier ribs 21. However, if the work substrate 11 is expanded, there is a likelihood that, as shown in FIG. 4, exposing of the phosphor layer forming layer is made even at the top of some of the barrier ribs 21, or not made in a region by the sidewall of the other rib. As a result, as shown in FIG. 5, the phosphor layer is formed at the top of the rib 21, but not formed by the wall of the other rib facing the rib 21, in the vicinity of the top thereof, rendering the shape of the phosphor layer asymmetrical from side to side in section. Accordingly, since magnitude of expansion of the work substrate 11 can not be estimated beforehand, the photomask 23 is designed to have openings such that the photomask protrudes inside the interval between the barrier ribs 21 as shown in FIG. 6 to prevent formation of the phosphor layer at the top of the barrier ribs 21 even if the photomask 23 is slipped out of place. Still, with the use of the collimated rays of light, the phosphor layers become asymmetrical in shape from side to side. Furthermore, when exposure is made via the photomask 23, it becomes difficult to expose the phosphor forming layer in a region by the sidewall of respective barrier ribs 21 because of the shadow of the photomask cast thereon. In practice, however, as the phosphor substance itself emits light in white color, the light rays are scattered, exposing the region by the sidewall of the rib as well. However, the phosphor layer near the sidewall of the rib is exfoliated when developed due to low intensity of the scattered light rays. This problem is addressed to at present by increasing light exposure, however, this entails the necessity of increasing the intensity of the light source.